<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .father {
            color: green;
        }
        /*中间有空格*/
        .father .father-son {
            color: red;
        }
        .father.uncle {
            font-size: 50px;
        }
        div.father{
            font-weight: 500;
        }
        div.mother{
            color: yellow;
            font-weight: 500;
        }
        div .mother-son {
            color: blue;
        }
    </style>
</head>
<body>
<div class="father uncle apple">
    <div class="father-son">
        father son
    </div>
    <div class="son-brother">
        son-brother
    </div>
</div>
<div class="mother">
    mother
    <span class="mother-son">
        mother son
    </span>
</div>
</body>
</html>